The present invention relates to satellite attitude control, and, more particularly to a method of compensating for the undesired effects of solar pressure or the like on the attitude of earth orbiting satellites.
It is generally desirable in communication satellite applications, to fix a satellite's position and orientation (attitude) with respect to certain celestial bodies. For example, in satellite communication applications it is common to position the satellite in a geostationary orbit around the earth while continuously orienting the satellite'solar collectors toward the sun and orienting the satellite's communications antennas toward the earth. This arrangement has the advantages of permitting multiple ground stations to continually use the satellite without interruption while the satellite gains the maximum benefit from the solar energy incident to its solar collectors.
The position and orientation of a satellite in a geostationary orbit must be occasionally adjusted because forces resulting from solar pressure, oblateness of the earth, and solar and lunar gravitational perturbations eventually alter the satellite's position and orientation beyond acceptable limits.
Several techniques have been developed for compensating for the disturbing effects on satellites. For example, gas powered thrusters can be briefly fired to return the satellite to its proper position and orientation whenever it deviates therefrom. While this method works well when relatively large infrequent corrective maneuvers are needed, such as northsouth stationkeeping, it is costly and inconvenient to frequently correct for smaller attitude disturbances, such as those created by solar pressure on the satellite's solar collectors.
Another technique which is frequently used to control a satellite's attitude employs the use of an on-board momentum wheel. Such wheels are usable in two modes--in a fixed orientation relative to the satellite, or on a gimballed platform. With a fixed momentum wheel, the satellite attitude errors are periodic when the external moments are periodic. The peak attitude errors are inversely proportional to the angular momentum of the wheel and directly proportional to the magnitude of the external moments. When there is a component of the external moment that has a fixed direction in space, or rotates with a yearly rather than a dialy period, the attitude errors will increase with time until they reach an unacceptable limit. At this point the attitude control thrusters must be employed to return the satellite's attitude within its desired range. In a typical satellite design, this use of the thrusters for attitude control is much more frequent than for orbit control, even though the fuel required for attitude control is relatively small.
With a gimballed momentum wheel, the satellite attitude errors due to external moments may be kept nulled even in the presence of external moments. This is accomplished by counteracting the external moments by the moments generated by gimballing the wheel, under the supervision of the attitude control system. For periodic external moments, the gimbal angles are periodic and their peak values are inversely proportional to the angular momentum of the wheel. A component of external moment that has a fixed direction in space or a yearly period will cause the gimbal angles to grow with time and eventually reach their limits. Before this occurs, the spacecraft thrusters must be employed to exert external moments which, by action of the spacecraft's attitude control system, return the momentum wheel gimbals to their desired operating range. This use of the thrusters for attitude control is much more frequent than for orbit control in a typical design, if no other means of obtaining control moments is available.
Still other techniques are used to generate control moments. One such technique is the use of magnetically induced torques to react against the earth's magnetic field. This method works reasonably well, but is subject to the vagaries of the earth's magnetic field at geosynchronous altitude. Another method is solar sailing, in which the solar radiation pressure reacting on the solar panels is used to create the desired compensating moments. The control moments generated by solar sailing are obtained by deviating the orientation of at least one of the satellite's solar panels slightly from the normal sun tracking position. When two panels are controlled in a coordinated motion, precise attitude control is possible in the fixed momentum wheel system. In the gimballed momentum wheel system, the amount of gimbal travel required for control may be substantially reduced. In both systems, the use of thrusters for attitude control is eliminated, except for the infrequent periods when they are used for orbit control. Solar sailing not only saves the fuel used to effect attitude control, but also enhances the reliability of the satellite, since the possibility of loss of control due to inadvertent firing of the trusters is minimized by locking out the thrusters during most of the mission.
The largest source of long period external disturbing moments is that produced when the center of solar radiation pressure does not coincide with the center of mass of the satellite, since this moment is fixed relative to the earth's sun line.
The general concept of single panel solar sailing is disclosed in U.S. Pat. No. 4,325,124, issued to Renner and entitled "System For Controlling the Direction of the Momentum Vector of a Geosynchronous Satellite." The Renner patent discloses a system for solar sailing wherein, depending on the direction of the disturbance torque, one solar collection array is rotated while the other panel remains in its normal sun tracking position. While the use of the Renner method does provide for some degree of satellite attitude control using solar sailing techniques, undesirably high windmill torques are induced on the satellite by virtue of rotating only one panel at a time. These undesired windmill torques inherently limit the overall attitude accuracy which can be attained when using the single-panel solar sailing technique for a fixed momentum wheel system, or result in excessive gimbal travel in a gimballed momentum wheel system.
Accordingly, it is a primary object of the present invention to provide a method for coordinated simultaneous rotation of both solar collection panels of a satellite to reduce the undesirable destabilizing effects of solar pressure and the like on a satellite's attitude.
Another object of the present invention is to provide a method of solar sailing as described above wherein satellite attitude is controlled without resort to the use of thrusters.
Another object of the present invention is to improve the attitude accuracy obtainable with a fixed wheel attitude control system.
Still another object of the present invention is to provide a method of solar sailing as described above which reduces the total gimbal travel of a gimbal-mounted momentum wheel used to compensate for the disturbing torques and thereby increase the useful life of the momentum wheel gimbal mechanisms.
Still another object of the present invention is to reduce the peak gimbal motion required of the gimbal mechanism, thereby simplifying its design.